Isoprene is an important monomer for the production of specialty elastomers including motor mounts/fittings, surgical gloves, rubber bands, golf balls and shoes. Styrene-isoprene-styrene block copolymers form a key component of hot-melt pressure-sensitive adhesive formulations and cis-poly-isoprene is utilised in the manufacture of tires (Whited et al., Industrial Biotechnology, 2010, 6(3), 152-163).
Manufacturers of rubber goods depend on either imported natural rubber from the Brazilian rubber tree or petroleum-based synthetic rubber polymers (Whited et al., 2010, supra).
Given a reliance on petrochemical feedstocks and the harvesting of trees, biotechnology offers an alternative approach via biocatalysis. Biocatalysis is the use of biological catalysts, such as enzymes, to perform biochemical transformations of organic compounds.
Accordingly, against this background, it is clear that there is a need for sustainable methods for producing intermediates, in particular isoprene, wherein the methods are biocatalysis based.
Both bioderived feedstocks and petrochemical feedstocks are viable starting materials for the biocatalysis processes.
The introduction of vinyl groups into medium carbon chain length enzyme substrates is a key consideration in synthesising isoprene via biocatalysis processes.
There are known metabolic pathways leading to the synthesis of isoprene in prokaryotes such as Bacillis subtillis and eukaryotes such as Populus alba (Whited et al., 2010, supra).
Isoprene may be synthesized via two routes leading to the precursor dimethylvinyl-PP, such as the mevalonate and the non-mevalonate pathway (Kuzuyama, Biosci. Biotechnol. Biochem., 2002, 66(8), 1619-1627). The mevalonate pathway incorporates a decarboxylase enzyme, mevalonate diphosphate decarboxylase (hereafter MDD), that introduces the first vinyl-group into the precursors leading to isoprene. The second vinyl-group is introduced by isoprene synthase (hereafter ISPS) in the final step in synthesizing isoprene.
The mevalonate pathway (FIG. 1) has been exploited in the biocatalytic production of isoprene using E. coli as host. E. coli engineered with the mevalonate pathway requires three moles of acetyl-CoA, three moles of ATP and two moles of NAD(P)H to produce a mole of isoprene. Given a theoretical maximum yield of 25.2% (w/w) for the mevalonate pathway, isoprene has been produced biocatalytically at a volumetric productivity of 2 g/(L·h) with a yield of 11% (w/w) from glucose (Whited et al., 2010, supra). Particularly, the phosphate activation of mevalonate to 5-diphosphomevalonate is energy intensive metabolically, requiring two moles of ATP per mole of isoprene synthesis (FIG. 1). Accordingly, reducing the ATP consumption can improve the efficiency of the pathway.